The present invention addresses the configuration of propellers, rotors, and windmills (including wind turbines), particularly those which must operate over a wide range of speeds and power settings. Such rotors must be able to vary both hub pitch and blade twist to avoid significant penalties in performance over much of their operating envelope. One method of the present invention described herein entails making the body of the blade flexible in twist, and using inertial torquers having relatively small fixed masses at one or more radial stations to generate twisting torques through centrifugal effect. With suitable choice of size, shape and location of the affixed masses in accordance with the present invention, a blade can be made to adopt near-ideal twist over a wide range of rotational speeds and hub-pitch angles.
The value of a variable-pitch, variable-twist rotor blade can be explained with reference to FIG. 3, which shows the airflow incident upon a typical blade section. For maximum rotor efficiency—that is, thrust generation with minimum power—the angle of attack α between the blade-section chord line 24 and the relative-wind vector V must fall within a narrow operating range, typically only a few degrees. If the angle of attack is outside of this range, then the blade will generate unnecessarily high drag, and correspondingly high power will be required to generate a desired thrust.
As FIG. 3 indicates, the relative-wind vector is the sum of a component Vt due to the blade's spin about the axis of the rotor, and a component Vx due to inflow along the spin axis of the rotor. For illustration, imagine a propeller-driven aircraft. At low forward speed, for example while accelerating for take-off, Vx is relatively small. Hence, the relative-wind vector makes a relatively shallow angle with the propeller's plane of rotation, and the incidence β of the blade section (i.e., the angle between the chord line and the plane of rotation) must likewise be shallow to put the angle of attack in the range for high efficiency. (The blade is then said to have “flat” or “fine pitch.”) At high forward speed, for example in cruise flight, Vx is relatively large. In this condition, the incidence of the blade must be made steeper if the ideal angle of attack is to be achieved. (The blade would then be said to have “coarse pitch”.)
Now consider the ideal variation of section incidence from hub to tip across the blade. The rotational component of inflow Vt is small at the hub and large at the tip. Meanwhile, the axial component Vx varies little across the blade. Hence, the inflow angle is steeper at the hub and shallower at the tip. If an ideal angle of attack is to be achieved on each blade section, then the blade must be twisted to match the inflow angle, with steeper incidence at the hub, and shallower incidence toward the tip. The ideal twist (i.e., difference in incidence between hub and tip blade sections) varies with the operating condition. An example is given in FIG. 5. The ideal twist (as well as the hub incidence) is relatively small at low forward speed (28), and relatively large at high forward speed (31).
Rotors which must operate over a wide range of speeds, as is typical for both propellers and windmills, would thus ideally have a mechanism to vary both hub incidence and blade twist in order to achieve optimum efficiency. But “fixed-pitch” propellers, as fitted to many aircraft, boats, and windmills, have no such mechanism. Their incidence is fixed, so they have high efficiency only in a narrow speed range. “Variable-pitch” propellers, which are also common, widen the range of efficient operation by varying hub pitch, but not twist. (That is, they have a mechanism to rotate the whole blade as a rigid unit about the hub.) An aircraft with a variable-pitch propeller typically has a better combination of climb and cruise performance than can be realized with fixed pitch. However, lack of a mechanism for varying twist can still be a large handicap.
The problem is particularly acute for aircraft such as “tilt-rotors” and “tail-sitters,” whose operating range extends from hover, with rotors thrusting vertically at zero axial speed, to forward flight, with rotors thrusting horizontally at high axial speed. Hover calls for flat hub incidence and little twist, while fast forward flight calls for coarse hub incidence and much more twist. Tilt-rotor aircraft built to date have variable-pitch rotors with fixed twist. As a result, even when hub incidence is adjusted for best performance in each condition, the inner part of a tilt-rotor blade may be left stalled in hover, and thrusting backward in forward flight. Rotor efficiency is thereby significantly penalized in both conditions.
Various methods have been proposed over the last half-century to avoid this penalty by making twist variable. These methods may be classified as either “active” or “passive.”
“Active” methods use mechanically-activated or electrically-activated devices along the blade to vary its shape. These include arrangements of torque-tubes and linkages, or of shape-memory or bistable materials, to adjust flexible or articulated blade sections. Such methods suffer from being heavy, costly, and complex to implement.
“Passive” methods are simpler. One such method is extension-twist coupling. A rotor blade can be built, for example with composite fibers laid at an appropriate angle to the spanwise axis, so that a change in axial tension causes twist. On some tilt-rotor aircraft, rotor speed and thus centrifugal tension is changed significantly between hover and forward flight. Hence, a blade of suitable construction can be made to change twist as desired between the two conditions. In practice, however, there may be little or no change in rotor speed between operating conditions requiring very different twist, as for example between hover and forward-flight climb. Extension-twist coupling is not effective in such situations.
Another mechanism for varying twist by centrifugal effect exploits the tendency of a set of spinning masses to maximize their aggregate moment of inertia about the spin axis. Thus, a mass element added to a blade will generate a torque that tends to align the principal axis of the element with the rotor's plane of rotation. The torque increases with the square of rotor speed. This “inertia-maximizing” effect from external bob-weights is used in various ways on propellers and windmills. Most notably, it is used for passive regulation of windmills, over a wide range of wind speed, without the need for a hub-pitch mechanism. Bob-weights affixed near the tips of torsionally-flexible blades cause twisting as rotor speed changes, and thus adjust the blade load. Another proposed use involves attaching actively-moveable bob-weights along the span of a torsionally-flexible blade. A servo would command changes in the location of each weight relative to the blade, and so modulate the inertia-maximizing torque in order to vary twist. As with other active techniques for blade twist variation, this involves cost and complexity in implementation.